1. Field of the Invention
The present invention relates to a motor control apparatus and a motor control method which are suitable for use in controlling a motor in an electric vehicle, an articulated robot system or the like. Particularly, the present invention enables an electric motor to generate a regenerative power with the suitable consumption.
2. Description of Related Art
The inventors of the present invention have proposed a two-wheel vehicle for assisting people movement (cf. e.g. Japanese Unexamined Patent Application Publication No. 2005-138631).
Further, a technique of consuming a regenerative power which is generated by a motor has been proposed (cf. e.g. Japanese Unexamined Patent Application Publication No. 2000-116190).
FIG. 13 shows the configuration of a vehicle having a wheel configuration (two wheels or three or more wheels) to which the present invention is applicable and employing inverted pendulum control with a main body having a gyro sensor and an acceleration sensor. Referring to FIG. 13, a step 2 on which a passenger stands is placed lower than the axle of left and right wheels 1L and 1R. The step 2 includes a posture sensor circuit 3 which has a gyro sensor and an acceleration sensor, and a central control device 5 which controls motors 4L and 4R that drive the wheels 1L and 1R according to the output of the posture sensor circuit 3.
The step 2 further includes a battery 6. The battery supplies a power to the posture sensor circuit 3, the motors 4L and 4R, and the central control device 5. Although the above description is given on the wheel configuration (two or more wheels) to which the present invention is applicable, the motor control apparatus and the motor control method of the present invention may be applied to other electric vehicle, articulated robot system and so on.
FIG. 14 is a block diagram showing the configuration of a control system according to a related art. Referring to FIG. 14, the signals which are detected by the gyro sensor 31 and the acceleration sensor 32 in the posture sensor circuit 3 are digitalized by analog-to-digital converters (ADCs) 33 and 34, respectively, and input to a central processing unit (CPU) 35. The information signal which is generated therein is transmitted to the central control device 5 through a serial input/output (SIO) 36.
In the central control device 5, the information signal from the posture sensor circuit 3 is received through an SIO 51. Further, angle information from a brake lever 52 and angle information from a turning lever 53 which controls the turning of a vehicle are supplied to ADCs 54 and 55, respectively, of the central control device 5. Based on the information, a CPU 56 and a memory 57 in the central control device 5 generate a torque command signal. The torque command signal is transmitted through SIOs 58 and 59 to control devices 41L and 41R of the motors 4L and 4R, respectively.
The motors 4L and 4R include encoders 42L and 42R, respectively, and the rotation of the motors 4L and 4R is fed back to the control devices 41L and 41R, thereby enabling stable control. Further, a secondary battery 61 is placed as the battery 6. For example, 24-volt direct current power from the secondary battery 61 is directly supplied to the control devices 41L and 41R, and it is also converted into 5-volt direct current power by a DC/DC converter 62 and supplied to the central control device 5.
FIG. 15 is a schematic diagram showing a typical motor connection. In this connection, one ends of motor coils 43U, 43V and 43W are connected with one another, and drive current of each phase is supplied to the other ends of the motor coils 43U, 43V and 43W. The phase current (Iu, Iv and Iw) is fed so that the sum total is (Iu+Iv+Iw=0), and the three-phase current with the current vector at a right angle with respect to the magnetic direction of a permanent magnet is fed to thereby generate rotation torque. In order to control the torque in this manner, it is necessary to control the three-phase current to have the following relationships:Iu=I0*sin(θm)[A]  Expression 1Iv=I0*sin(θm+2π/3)[A]  Expression 2Iw=I0*sin(θm−2π/3)[A]  Expression 3where θm is a rotor magnetic pole position [rad] with respect to a stator. The current is thereby controlled in such a way that a current axis and a magnetic pole axis make a right angle, using the Fleming's left-hand rule.
FIG. 16 is a block diagram of an inverter circuit to generate the three-phase current having the relationship of the above expressions 1 to 3. Referring to FIG. 16, pulse width modulation (PWM) signals PWMu, PWMv and PWMw, which correspond to phase current (Iu, Iv, Iw), are supplied to field-effect transistors (FETs) 44U, 44V and 44W which are placed between one ends of the motor coils 43U, 43V and 43W and a ground, respectively, and also supplied to FETs 46U, 46V and 46W of which phases are shifted through inverters 45U, 45V and 45W and which are placed between one ends of the motor coils 43U, 43V and 43W and a power supply. Such a circuit configuration enables the three-phase current having the relationship of the above expressions 1 to 3 to flow.
The current vector control of the three-phase coils of an AC motor can be equivalently performed in the same manner as that of a DC motor. FIG. 17 shows a mathematical model of a motor coil of the equivalent DC motor. Referring to FIG. 17, a speed control command is supplied to a speed feedback gain 402 through a subtracter 401 and further supplied to a current control gain 404 through a subtracter 403. The current value is converted into a DC±24 volt power in an output amplifier 405 and it is supplied through an adder 406 as a coil voltage to a motor coil 407.
The current of the motor coil 407 is fed back to the subtracter 403 and also produces torque through a motor constant 408. The torque is acquired as a rotation speed (motor output) through a motor rotor/load moment 409. The rotation speed is fed back to the adder 406 through a motor back-electromotive force coefficient 410 and also fed back to the subtracter 401. The feedback control of motor driving is performed in this manner. FIG. 17 shows the coefficient of each component.
When such a motor is used in a vehicle, a robot or the like, if a driving target decelerates and a mechanical energy due to an inertial force is transferred to the motor, the motor converters the mechanical energy into an electric energy by the Fleming's right-hand rule, thereby generating a regenerative power. Because the motor thereby acts as an electric generator, a power supply voltage of the inverter circuit increases. It is thereby necessary to connect an external regenerative resistor to a power supply as shown in FIG. 18 so as to convert a regenerative power energy into a thermal energy by the regenerative resistor in order to suppress an increase in a voltage.
FIG. 19 shows a speed feedback system using a motor. As a feedback signal, a gyro sensor signal and an acceleration sensor signal, in addition to a speed signal, can be treated in the same manner. FIGS. 20A to 20E are waveform charts showing the control signals when a command which causes a motor to start rotating and then to stop after a certain period of rotation is given in the control system of FIG. 19. Specifically, FIG. 20A shows a motor speed command, FIG. 20B shows a motor speed, FIG. 20C shows an amplifier voltage output, FIG. 20D shows a motor back-electromotive force, and FIG. 20E shows a motor coil voltage.
As shown in the motor coil voltage in FIG. 20E, a coil voltage is such a voltage that a back-electromotive force is added to a power supply voltage upon deceleration. Thus, a voltage that is two times larger than a power supply voltage at maximum is generated during deceleration. Accordingly, if a motor is used in a system such as a vehicle, the motor acts as an electric generator when a vehicle moves down a downward slope, so that a voltage undesirably increases to exceed a withstand voltage. It is therefore necessary to create a mechanism for converting a kinetic energy into a thermal energy by a regenerative resistor in order to suppress an increase in a voltage.
In the system of FIG. 18, the coil voltage which is output from the adder 406 is supplied to a regenerative voltage detection circuit 411 and a regenerative voltage is thereby detected. When the regenerative voltage becomes higher than a prescribed value, a switching device 412 becomes conductive, so that an excessive regenerative voltage is supplied to a regenerative resistor or capacitor 413. In this manner, in a related art, a regenerative power is converted into a thermal energy by the regenerative resistor, or a regenerative energy is stored in a regenerative capacitor, thereby suppressing an increase in a voltage.
FIGS. 21A to 21E are waveform charts showing the signals in the case of using the circuit configuration of FIG. 18. Specifically, FIG. 21A shows a motor speed command, FIG. 21B shows a motor speed, FIG. 21C shows an amplifier voltage output, FIG. 21D shows a motor back-electromotive force, and FIG. 21E shows a motor coil voltage. As shown in the motor coil voltage in FIG. 21E, the fluctuation of a power supply voltage due to a back-electromotive force is suppressed. FIG. 22A shows the waveform of generated regenerative current, and FIG. 22B shows the waveform of the motor coil voltage. FIGS. 22A and 22B show that the fluctuation of the motor coil voltage is smaller than the generated regenerative current.
Thus, in the configuration of an apparatus according to a related art, an increase in a power supply voltage is suppressed with the use of the built-in regenerative resistor or capacitor 413 as shown in FIG. 13 or 14. However, the regenerative resistor or capacitor 413 has a large weight and volume. This causes an increase in the mass or size of a vehicle, robot or the like in which the above motor is used.
As described in the foregoing, a related art uses a regenerative resistor or a capacitor with a large weight and volume in order to suppress an increase in a power supply voltage. This hinders the achievement of size or weight reduction of a vehicle, robot or the like in which the motor is used. In light of the foregoing, there is a need for eliminating a generated regenerative power without the use of a regenerative resistor or a capacitor.